This invention relates generally to magnetic recording, and more particularly to head indexing devices for use with magnetic tape.
It is known to record data on parallel tracks on a recording medium such as magnetic tape. It is also known to access these various tracks by moving a transducer head across the tape to selectively access a given track. U.S. Pat. No. 4,275,427 dated June 23, 1981 to J. Bjordahl discloses one such system to which attention is directed.
U.S. Pat. No. 3,839,735 dated Oct. 1, 1974 to J. P. Denk discloses yet another such system to which attention is directed.
The use of parallel high density tracks (center line to center line of the tracks being less than 0.020 inches) on the magnetic tape requires the precise reproducible indexing of the transducer head to ensure that the head is accessing the proper track and is properly aligned with it. In the past this has involved the use of an electric stepper motor having sufficient torque (e.g. greater than two ounce-inches), having very small step angles (approximately 1.8.degree.), and having a precision (no backlash) linkage mechanism between the motor and the head. This linkage mechanism had to be precision made so as to enable precise control of the head position.
As well as the precision involved in the individual components (i.e. the motor, the linkage mechanism, and the head) the mounting, the interconnecting, and the adjustment of all these components also required a great degree of precision.
Past designs have been limited in their accuracy by problems such as the following:
1. The total run out of the electric shaft can produce an error of approximately 0.002 inches.
2. The end play of the electric motor shaft can produce an error of approximately 0.001 inches or more.
3. The angular indexing change due to friction in the system can cause an index error of approximately 0.001 inches.
4. The angular indexing change due to inertia in the system can cause an index error of approximately 0.001 inches.
5. The step accuracy of the stepper motor can cause an error of approximately 0.001 inches.
6. The pitch accuracy of the screw or spiral cam can cause an error of approximately 0.0005 inches.
7. Due to the variety of materials used, a variation in the temperature (e.g. 50.degree. F. to 100.degree. F.) can cause an indexing error of approximately 0.002 to 0.004 inches (depending upon materials).
8. Manufacturing difficulties of reproducibly setting up the stepper motor to match with the desired home position can cause an initial error of 0.002 inches or more.